1. Field of the Invention
The present invention relates to a method of producing microstructured surface reliefs, in which the surface relief is embossed with an embossing device into a thixotropic coating composition applied to a substrate; to substrates provided with this microstructured surface relief; and to the use of these substrates.
2. Discussion of Background Information
Surface relief structures are used for various fields of application. At the forefront stand decorative applications, on metal, plastic, card or stone, for example. Additionally, applications for producing nonslip floor coverings, footwear soles, finished textiles, structured soundproofing panels or electrical cables are specified. Methods used to produce relief structures with dimensions in the mm range include not only screen printing but also printing with structured rollers or casting. Factors governed by the application technology dictate the use of thixotropic, pseudoplastic or high-viscosity coating materials, with thixotroping being effected using additives known from the prior art. Said additives may include fine-scale inorganic powders, such as SiO2 or CaCO3. Thixotropic coating systems and binder systems may also be used to produce stochastic surface relief structures by way of spraying methods, with the addition of relatively coarse particles which determine the structural geometry.
An important part is played by roller embossing methods. A distinction is made here between hot embossing, the embossing of thixotropic coating materials, and reactive embossing. In the case of hot embossing, the embossing roll is pressed into a thermoplastic substrate which has been heated to above the glass transition point. After the roll has been withdrawn the structure is fixed by rapid cooling. Using small-sized, rigid dies, this method is also being investigated analogously for producing very fine structures in the μm and 100 nm range for electronic applications. Disadvantages here are inaccuracies, caused by the high thermal expansion coefficients of the thermoplastic polymers used, and the high restoring forces due to very small radii of curvature, which lead to rounding off of edges even on rapid cooling. Further disadvantages are the long process times and also the fundamental unsuitability for what is known as stepping, in which large areas are structured by a sequence of embossing operations on adjacent unit areas using a small die which is offset in steps. In the embossing of thixotropic coating materials, the thixotropic rheology of the coating material means that the relief is substantially retained, at least for a certain time, within which fixing can take place by curing or drying. To date, however, this method has been used only for producing relatively coarse structures with dimensions in the mm range.
In the case of structures with dimensions in the μm to nm range for optical or microelectronic applications, the faithfulness of reproduction is subject to very high requirements. Optical and microelectronic μm or nm structures therefore require near-net shaping with defined sidewall steepness.
Besides hot embossing, only reactive embossing has been used for surface relief structures with dimensions in the μm to nm range. In reactive embossing, it is vital that the structured coating film beneath the planar die used is cured by thermal treatment or UV irradiation before the impressed die can be removed from the coating film. This is also the case when further compaction takes place by a further, downstream temperature treatment. A. Gombert et al., Thin Solid Films, 351 (1,2) 1999, 73-78, assume that, even in the case of transfer of reactive embossing to the roller technology, curing must take place under the embossing die. The assumption is made that this is necessary in order to prevent the surface forces of the uncured layer, which are particularly high at small radii of curvature, from leading to rounding of the microstructure and thus to a loss of reproduction faithfulness in any attempt at thixotropic embossing. From a technological standpoint, however, curing following removal of the roll would be of particular interest, since it would allow surface reliefs on large areas, e.g., as motheye antireflection structures for display applications, to be produced by the roller method in a shorter and more reliable process than with curing under the roll.
The object on which the invention is based is therefore to provide a method of producing microstructures with dimensions in the lower μm to nm range which on the one hand ensures the stringent reproduction faithfulness requirements required in this dimensional range and on the other hand allows shorter production times.